1. Field of the Invention
This invention relates generally to a power supply circuit for a motor vehicle and more particularly to a power supply circuit for a motor vehicle with two different load voltages.
2. Discussion of the Background
The usual power supply circuit for motor vehicles comprises a three-phase ac generator consisting of a rotor with a dc current and a three-phase coil arranged in the stator. The three-phase voltage produced during the rotation of the rotor is converted into a pulsating dc voltage in a rectifier bridge and fed to the vehicle supply system. A voltage adjustment to the maximum charging voltage of 14.2 V for the battery is required. This voltage control is performed by the voltage controller or regulator, which periodically disconnects the exciter current in the rotor.
The usual vehicle supply system voltage of 12 V is inadequate for some specific electrical loads, e.g. for thin-film window heating glazing or window defogging strips. These require a voltage of 50 V or more to achieve the required effect. Since the other loads require a vehicle supply system voltage of 12 V, it is necessary to have two different voltage systems.
One possible method of creating two different voltages is to separate the generator from the vehicle supply system and adjust it to the higher voltage required for the window heating glazing with the vehicle supply system being supplied by the battery during this period. With this circuit, however, the battery must fully takeover the vehicle system supply while the window heating glazing is switched on, so that it could be quickly discharged. Since the vehicle will not operate if the vehicle voltage is lost, an elaborate monitoring circuit must be installed to monitor the condition of the battery. Consequently, the circuit is relatively expensive. In addition, the life of the battery is shortened as a result of its rapid discharge.
Another possible method of producing the two different voltages is to generate the higher voltage required for operation of the window heating glazing from the vehicle supply system voltage. For this purpose, a dc voltage converter is required, with the vehicle supply system dc voltage first being converted into an ac voltage, the latter transformed to the higher voltage level and then rectified again.
However, step-up converters of this type have poor efficiency, particularly because of the high voltage drop at the semiconductor switch on the primary side. Because of the higher power to be transmitted in the case of window heating glazing, converters of this type are also large and expensive.